Additives for Reduction of Exhaust Emissions from Compression Ignition Engines

ABSTRACT

Exhaust emissions resulting from the combustion of hydrocarbon fuels in compression ignition engines may be reduced using a homopolymer that may be polyisobutylene, polypropylene, and/or hyperbranched polyalpha-olefins. The homopolymer may have a molecular weight of from about 1600 to about 275,000. Optionally, an alkyl nitrate such as 2-ethylhexylnitrate (2EHN), and/or a peroxide, such as hydrogen peroxide, may also be used together with the homopolymer. Both NOx and particulate matter emissions (PM) may be reduced using ppm quantities of the additive compositions; alternatively, NOx emissions may be lowered or reduced while PM emissions do not substantially increase.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application from U.S. patentapplication Ser. No. 12/128,918, filed May 29, 2008, which claims thebenefit of U.S. Provisional Patent Application No. 60/940,914 filed May30, 2007.

TECHNICAL FIELD

The present invention relates to additives for distillate fuels, andmore particularly relates, in one embodiment to reducing exhaustemissions for hydro-carbon fuels using chemical additives.

TECHNICAL BACKGROUND

It is well known that considerable effort has been expended reducing theexhaust emissions from compression ignition (e.g. internal combustion)engines. These exhaust emissions are the products of burning the fuel inthe engine, emitted from an exhaust system. The major emissions includehydrocarbons, which are unburned or partially burned fuels, nitrogenoxides (generally abbreviated NOx) which are generated when nitrogen inthe air reacts with oxygen under the high temperature and pressureconditions inside the engine, carbon monoxide (CO) which is a product ofincomplete combustion, and carbon dioxide (CO₂) which is a product ofthe complete combustion of hydrocarbons.

Additives to fuels are known to reduce undesirable emissions. There aremany fuel additives that claim to lower emissions, such as particulatematter, unburnt hydrocarbon, and NOx. Various organo-metallic andtotally organic formulations have been proposed and tried. Furthermore,diverse mechanisms have been proposed for their effectiveness.

It has been found to be difficult to simultaneously reduce particulatematter (PM) emissions and NOx emissions, particularly in diesel fuels.Unfortunately, with some additives, as the PM is lowered, NOx emissionsrise, and vice versa with others. There is some promise that ethanolfuel additives may help reduce both PM and NOx simultaneously undercertain conditions.

Thus, it would be desirable if other additives could be developed toreduce the emissions of distillate fuels upon combustion.

SUMMARY

There are provided, in one non-limiting form, compositions for reducingthe emissions of distillate fuels that includes a homopolymer such aspolyisobutylene (PIB), polypropylene (PP), and/or a hyperbranchedpolymer, where the homopolymer has a molecular weight of from about 1600to about 275,000. Combinations of these polymeric materials with analkyl nitrate, such as 2-ethyhexylnitrate (2EHN), and/or a peroxide,such as hydrogen peroxide, are also useful.

There are further provided in another non-restrictive version distillatefuels, such as diesel fuels, gasoline, jet fuels, or kerosene, havingreduced emissions, that contains an effective amount of a composition toreduce emissions of a homopolymer that may be polyisobutylene,polypropylene, and/or a hyperbranched polymer, where the homopolymer hasa molecular weight of from about 1600 to about 275,000, and optionallyan alkyl nitrate and/or a peroxide.

Also provided in another non-limiting embodiment are methods forreducing emissions of a distillate fuel by adding to the fuel aneffective amount of a composition that includes a homopolymer that maybe polyisobutylene, polypropylene, and/or a hyperbranched polymer, wherethe homopolymer has a molecular weight of from about 1600 to about275,000, and optionally an alkyl nitrate and/or a peroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of NOx emissions in a distillate fuel showing NOxreduction measured as % as a function of total concentration of theadditives herein;

FIG. 2 is a graph of particulate matter emissions measured as a functionof the total concentration of the additives for the fuels of FIG. 1;

FIG. 3 is another graph of NOx emissions in a distillate fuel showingNOx reduction measured as % as a function of total concentration of anadditive composition containing 10% hyperbranched polyalpha-olefin(HPAO) and 70% 2-ethylhexyl nitrate (EHN), the balance being solvent;and

FIG. 4 is a graph of particulate emissions (PM) in a distillate fuelshowing NOx reduction measured as % as a function of total concentrationof an additive composition containing 10% hyperbranched polyalpha-olefin(HPAO) and 70% 2-ethylhexyl nitrate (EHN), the balance being solvent.

DETAILED DESCRIPTION

The methods and compositions herein relate to reducing the amount ofexhaust emissions resulting from the combustion of hydrocarbon fuels incompression ignition engines such as internal combustion engines. Inparticular, the additives reduce NOx emissions and/or particulatematter. More specifically, the methods and compositions herein concern afuel additive formulation that includes a polymer. Suitable polymers arehomopolymers including, but not necessarily limited to, polyisobutylene,polypropylene, hyperbranched polymers, and in particular hyperbranchedpolyalpha-olefins (PAO), and the like. In one non-restrictive version,the hyperbranched polyalpha-olefins may be hyperbranched polymers ofC4-C30 alpha-olefins, where the alpha-olefins may be acid- oralcohol-functionalized, and mixtures and derivatives thereof. In onenon-limiting embodiment, the polymer presence lowers NOx and in manyembodiments also lowers particulate matter (PM).

The additive composition herein may also optionally contain a componentthat may be an alkyl nitrate and/or a peroxide. Suitable alkyl nitratesinclude, but are not necessarily limited to, 2-ethylhexyl nitrate(2EHN), CH₃(CH₂)₃CH(C₂H₅)CH₂ONO₂, iso-propyl nitrate, iso-amylnitrate,iso-hexylnitrate, cyclohexyl nitrate, dodecyl nitrate, diglycol nitrateand tetraglycol nitrate and the like. Ether nitrates and fatty acidnitrates may also be useful. The alkyl nitrate may function to primarilylower the NOx emissions although reduction in PM may also be expected.Alternatively, NOx emissions may be lowered by the compositions hereinwithout appreciably raising PM levels, which would also be an advantageand an improvement.

The additive composition may also optionally include a peroxide, inplace of or in addition to the alkyl nitrate. Suitable peroxidesinclude, but are not necessarily limited to, hydrogen peroxide,di-tertiary butyl peroxide, and benzoyl peroxide and the like. Further,some synergism has been found between the homopolymer and the alkylnitrate and/or peroxide. Known cetane boosters for use in distillatefuels include 2-ethylhexyl nitrate, tertiary butyl peroxide, diethyleneglycol methyl ether, cyclohexanol, and mixtures thereof. Conventional,known ignition accelerators include hydrogen peroxide, benzoyl peroxide,di-tert-butyl peroxide, and the like.

By hyperbranched polyalpha-olefins are meant polymers prepared bypolymerizing hydrocarbons under free radical conditions at lowpressures. Suitable free radical catalysts include, but are notnecessarily limited to, metallocenes and transition metal catalysts,along with peroxide catalysts and Ziegler-Natta catalysts. The polymersare unique in that although hydrocarbon polymers generally have highermolecular weight, greater viscosity and greater hardness than thestarting hydrocarbon these polymers generally have higher melting pointsand congealing points than the starting hydrocarbons. The hydrocarbonsemployed are primarily alpha-olefins of the formula RCH═CH₂ but may alsoinclude alpha-olefins having vinylidene structures, internal olefins andsaturates, where R is an alkyl or alkylene group, including those havingvinylidene structures. Suitable hyperbranched polyalpha-olefins arethose made according to the methods described in U.S. Pat. Nos.4,060,569; 4,239,546 and 6,776,808, all incorporated by reference hereinin their entirety. The hyperbranched polyalpha-olefins are consideredhomopolymers herein under the classic definition because they are madefrom a single monomer. Suitable hyperbranched polyalpha-olefins hereinmay have a number average molecular weight (M_(n)) of from about 100 toabout 275,000, alternatively a lower threshold of about 150 andindependently an upper threshold of about 250,000, and in anothernon-limiting embodiment from about 200 independently up to about175,000, or even up to about 125,000. Alternative lower thresholds to beused within these ranges include, but are not necessarily limited to,about 1600, about 1700 and about 1800, even about 2000. The patentsnoted above do describe copolymers which are not encompassed by theadditive compositions and methods herein.

Hyperbranched polyalpha-olefins have a unique physical and chemicalstructure compared with conventional homopolymers of ethylene,propylene, butylene (1- or 2-), pentylene or isobutylene. Hyperbranchedpolyalpha-olefins have long alkyl groups on tertiary carbons and“branches on branches”. By “long alkyl groups” is meant alkyl groups offrom 4 to 50 carbon atoms; alternately from 4 to 24 carbon atoms, and inanother non-limiting embodiment, from 4 to 14 carbon atoms.Hyperbranched polyalpha-olefins are expected to have at least two alkylbranches on at least two other alkyl branches, whereas conventionalhomopolymers noted above have no such “branching on branching”. This isin contrast to polyisobutylene, which at most has methyl “branches”.Indeed, the maximum alkyl branch length from the conventionalhomopolymers in the list above is C3, and again, they have no brancheson branches.

Other polymers that may also be useful in the additive compositionsherein include, but are not necessarily limited to, isotacticpolypropylene (such as ones having a weight average molecular weight inthe range of about 1600 to about 2000; alternatively about 1700 to about2000) or higher molecular weight hyperbranched polymer products thanthose described above. Polymer alone without the 2EHN may be useful.

Suitable homopolymers include, but are not necessarily limited topolyisobutylene, polypropylene, hyperbranched polymers, and mixturesthereof, where the homopolymer has a M_(n) molecular weight of fromabout 1600 to about 275,000; alternatively the lower M_(n) threshold isabout 1600, about 1700, about 1800 or about 2000, where alternativelythe upper threshold, in combination with any of the lower thresholds,may be about 275,000, about 250,000, about 175,000, or about 125,000 togive acceptable alternative M_(n) ranges.

The methods herein relate to additive compositions for distillate fuels,as contrasted with products from resid. In the context herein,distillate fuels include, but are not necessarily limited to dieselfuel, kerosene, gasoline, jet fuel, and the like. It will be appreciatedthat distillate fuels include blends of conventional hydrocarbons meantby these terms with oxygenates, e.g. alcohols, such as methanol,ethanol, and other additives or blending components presently used inthese distillate fuels, such as MTBE (methyl-tert-butyl ether), or thatmay be used in the future. In one non-limiting embodiment herein,distillate fuels include low sulfur fuels, which are defined as having asulfur content of 0.2% by weight or less, and in another non-limitingembodiment as having a sulfur content of about 0.0015 wt. % or less—suchas the so-called “ultra low sulfur” fuels. Particularly preferredhydrocarbon fuels herein are diesel and kerosene. It is expected that amore conventional diesel fuel (i.e. with an aromatic content of >28%)treated with the additive composition herein will be equivalent inemissions to a Texas Low Emissions Diesel (TxLED) fuel with <10%aromatic content.

Generally, in one non-limiting embodiment herein the composition forimproving the emissions of distillate fuels is a mixture or blend of2EHN (or a peroxide component) and at least one of the homopolymers. Inanother non-restrictive version herein the homopolymer is present in thefuel in the range of about 20 to about 2500 ppm, in one non limitingembodiment from about 20 independently up to about 300 ppm;alternatively from about 20 independently up to about 150 ppm. The alkylnitrate, particularly 2EHN, may be present in the fuel in the range ofabout 100 to about 3000 ppm, alternatively from about 500, independentlyup to about 1500 ppm. In one non-limiting embodiment, the volume ratioof homopolymer to the component ranges from about 1:1 to about 1:100,and alternatively the volume ratio of homopolymer to the componentranges from about 1:2 to about 1:10; and in one particularly suitableratio, about 1:7.

Typically, a solvent may be advantageously used in the compositionsherein, where the solvent may be aromatic solvents and pure paraffinicsolvents. Aromatic solvents are particularly preferred. The proportionof solvent in the total fuel additive composition may range from about 0to 90 weight %; in another non-restrictive embodiment, the solvent mayrange from a lower threshold of about 15 wt % independently to an upperthreshold of 45 wt %. The use of a solvent is optional. In somenon-limiting embodiments, no solvent is used or desired. Anon-restrictive example would be 87.5% 2EHN and 12.5% HPAO with nosolvent (a 7:1 ratio of active components). Specific examples ofsuitable solvents include, but are not limited to paraffins andcycloparaffins, aromatic naphtha, kerosene, diesel, gasoline, xylene,toluene, alcohols (e.g. 2-ethylhexanol), and the like.

It will be appreciated that the methods and compositions herein alsoencompass distillate fuels containing the additive compositionsdescribed herein, as well as methods of improving the emissionsproperties of distillate fuels using the additive compositions describedherein.

Other, optional components of the distillate fuels in non-limitingembodiments may include, but are not necessarily limited to detergents,pour point depressants, cetane improvers, lubricity additives, dehazers,cold operability additives, conductivity additives, biocides, dyes, andmixtures thereof. Particularly useful components may includecondensation reaction products of aldehydes and amines which are usefulas antioxidants and are effective to lower PM and unburnt hydrocarbon(HC). A specific non-limiting example is the condensation reactionproduct between formaldehyde and di-n-butylamine. In anothernon-limiting embodiment, water is explicitly absent from the additivecomposition.

The invention will be illustrated further with respect to the followingnon-limiting Examples that are included only to further illuminate theinvention and not to restrict it.

Examples 1-3

Additive compositions expected to be useful herein include, but are notnecessarily limited to the following outlined in Table I:

TABLE I Fuel Additive Compositions to Reduce Exhaust Emissions Ex.Polymer 2EHN Solvent Other 1 10 wt % PIB 70 wt % 15 wt % aromatic 5 wt %2-ethylhexanol 2 10 wt % hyper- 70 wt % 15 wt % aromatic branchedpolymer 5 wt % 2-ethylhexanol 3 70 wt % 10 wt % aromatic 20 wt % ProductQ The hyperbranched polymer is a polyalpha-olefin having a molecularweight of about 2800. The 2-ethylhexanol (2EH) was added as a solvent toimprove the low temperature stability of the additive formulation.Product Q is a condensation reaction product between formaldehyde anddi-n-butylamine.

Examples 4-9

Other additive compositions expected to be useful herein include, butare not necessarily limited to those outlined in Table II:

TABLE II Component Description Wt-% Ex. 4 PIB, 1500 MW 10 2-Ethylhexylnitrate 70 Aromatic solvent 15 2-Ethylhexanol 5 Ex. 5 Hyperbranched PAO10 2-Ethylhexyl nitrate 70 Aromatic solvent 15 2-Ethylhexanol 5 Ex. 62-Ethylhexyl nitrate 70 bis-(dibutyl)diaminomethane 20 Aromatic solvent10 Ex. 7 PIB, 1500 MW 50 Aromatic solvent 45 2-Ethylhexanol 5 Ex. 8Hyperbranched PAO 50 Aromatic solvent 45 2-Ethylhexanol 5 Ex. 9 PIB,1500 MW 10 2-Ethylhexyl nitrate 70 Aromatic solvent 10 Polyester diol(for lubricity) 5 2-Ethylhexanol 5

The test data in the Figures discussed below was developed using a 1991DDC Series 60 (Serial No. 06R0038671) heavy duty diesel engine mountedin a transient-capable test cell. This engine had an in-line, sixcylinder configuration rated for 365 hp at 1800 rpm, was turbocharged,and used a laboratory water to air heat exchanger for a charge airintercooler. The exhaust was routed to a full flow constant volumesampler that utilized a positive displacement pump. Total flow in thetunnel was maintained at a nominal flow rate of about 2000 SCFM. Samplezone probes for particulate matter (PM), heated oxides of nitrogen(NOx), heated hydrocarbons (HC), carbon monoxide (CO), and carbondioxide (CO2) measurements were connected to the main tunnel. Probes forbackground gas measurement were connected downstream of the dilution airfilter pack, but upstream of the mixing section. The dilution system wasequipped with pressure and temperature sensors at various locations inorder to obtain all necessary information required by the U.S. Code ofFederal Regulation (40 CFR, Part 86, Subpart N).

FIG. 1 depicts the NOx mitigation that is achieved with variousformulations of HPAO and PIB, alone and in combination with EHN in acompression ignition fuel. The y-axis indicates the percent NOxreduction. The x-axis indicates the total concentration in ppm (wt.) ofthe additive component or components for a particular test. FIG. 1illustrates several points:

-   -   1. All of the formulations mitigated NOx, at least to some        extent.    -   2. PIB alone performed considerably better than HPAO alone.    -   3. The combinations of polymer plus EHN performed better than        polymer alone or EHN alone.    -   4. The HPAO plus EHN combination performed better than the        corresponding combination with PIB, in spite of the fact than        PIB alone performed better than HPAO alone.    -   5. The HPAO plus EHN combination performed better at 2000 ppm        than EHN alone at 3000. This point becomes even more significant        when the fact is considered that the combination product at 2000        ppm has a total of 1600 ppm active components (1400 EHN and 200        HPAO). This clearly indicates a synergism between HPAO and EHN.

FIG. 2 depicts the PM emissions that were achieved with the distillatefuels and the additives of FIG. 1. HPAO and EHN alone did not affect PM.PIB alone gave slightly increased PM (slightly negative reduction). Itmay be noted that at about 1600 ppm total concentration, the fuel with1400 ppm EHN and 200 ppm PIB had improved PM reduction. At about 2400ppm total concentration, 2100 ppm EHN and 300 ppm PIB, the PM emissionsincreased (negative reduction). At about 1600 ppm total concentration,1400 EHN and 200 ppm HPAO gave somewhat increased PM emission, but atabout 2400 ppm total concentration, 2100 ppm EHN and 300 PIB gave no PMchange.

FIG. 3 depicts data on one formulation, 10% HPAO/70% EHN in sixdifferent distillate fuels that met ASTM D975 specifications, but variedin composition, at various dosages. The y-axis is the same as in FIG. 1,but the x-axis is ppm of the additive as formulated. In every case theeffectiveness of the additive is clear, but the degree of effectivenessvaries from fuel to fuel. For instance, at an additive dosage of 2500ppm (as formulated) NOx reduction was as high at 7% in the fuel with thebest response and as low as about 3% in the fuel with the worstresponse.

In addition to the effectiveness of the HPAO and HPAO-EHN combinationsin mitigating NOx, there is clear evidence that these components do sowithout increasing particulate matter to any significant extent and, infact, in most cases it actually lowers PM. FIG. 4 is illustrative ofthis point. In two of the three fuels PM was mitigated by as much asabout 7%, whereas in one fuel there was a very slight, and most likelynot statistically significant, increase in PM. Another different fuel,Fuel E, gave a more pronounced negative reduction in PM (increase in PM)than Fuel C. Without being limited to any particular explanation, it maybe that this Fuel E behavior was due to high aromatics content and anunusually high specific gravity.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been demonstrated aseffective for reducing the emissions of fuels. However, it will beevident that various modifications and changes can be made theretowithout departing from the broader spirit or scope of the invention asset forth in the appended claims. Accordingly, the specification is tobe regarded in an illustrative rather than a restrictive sense. Forexample, specific combinations of polymers optionally together withalkyl nitrates and/or peroxides falling within the claimed parameters,but not specifically identified or tried in a particular composition toimprove the emissions of fuels herein, are expected to be within thescope of this invention. Certain compositions under certain conditionsmay serve to lower NOx emissions without any substantial increase in PMemissions or with substantially unchanged PM emissions. It isanticipated that the compositions of this invention may also impart tothe engines in which they are used as emissions reducers, greaterhorsepower, and better fuel economy as a result of less friction,whether they are used in diesel or gasoline engines.

The present invention may suitably comprise, consist or consistessentially of the elements disclosed and may be practiced in theabsence of an element not disclosed.

The words “comprising” and “comprises” as used throughout the claims isto interpreted “including but not limited to”.

What is claimed is:
 1. A method for reducing emissions of a distillate fuel comprising adding to the distillate fuel an effective amount of an additive composition for reducing the emissions of the distillate fuel, the additive composition comprising a homopolymer selected from the group consisting of polyisobutylene, polypropylene, hyperbranched polymers, and mixtures thereof, where the homopolymer has a molecular weight of from about 1600 to about 275,000.
 2. The method of claim 1, where the additive composition further comprises a component selected from the group consisting of an alkyl nitrate, a peroxide and combinations thereof.
 3. The method of claim 2 where the effective amount of the component ranges from about 100 to about 3000 ppm and the effective amount of the homopolymer ranges from about 20 to about 2500 ppm, both based on the total distillate fuel.
 4. The method of claim 1 where the fuel has reduced NOx and/or particulate matter emissions as compared to an otherwise identical fuel absent the additive composition.
 5. The method of claim 1 where the fuel has reduced NOx and particulate matter emissions are substantially the same as or lower compared to an otherwise identical fuel absent the additive composition.
 6. A method for reducing emissions of a distillate fuel comprising adding to the distillate fuel an additive composition comprising: from about 20 to about 2500 ppm, based on the total distillate fuel, of a homopolymer selected from the group consisting of polyisobutylene, polypropylene, hyperbranched polyalpha-olefins, and mixtures thereof, where the homopolymer has a molecular weight of from about 1600 to about 275,000; and from about 100 to about 3000 ppm, based on the total distillate fuel, of a component selected from the group consisting of an alkyl nitrate, a peroxide and combinations thereof.
 7. The method of claim 6 where the fuel has reduced NOx and/or particulate matter emissions as compared to an otherwise identical fuel absent the additive composition.
 8. The method of claim 6 where the fuel has reduced NOx and particulate matter emissions are substantially the same as or lower compared to an otherwise identical fuel absent the additive composition.
 9. An additive composition for reducing the emissions of distillate fuels comprising: a homopolymer selected from the group consisting of polyisobutylene, polypropylene, hyperbranched polymers, and mixtures thereof, where the homopolymer has a molecular weight of from about 1600 to about 275,000; and a component selected from the group consisting of an alkyl nitrate, a peroxide, and combinations thereof.
 10. The composition of claim 9 where the volume ratio of homopolymer to the component ranges from about 1:1 to about 1:100.
 11. The composition of claim 9 where the component is an alkyl nitrate selected from the group consisting of 2-ethylhexyl nitrate (2EHN), iso-propyl nitrate, iso-amylnitrate, iso-hexylnitrate, cyclohexyl nitrate, dodecyl nitrate, diglycol nitrate and tetraglycol nitrate.
 12. A distillate fuel comprising: a hydrocarbon selected from the group consisting of diesel fuel, gasoline, jet fuel, and kerosene; and an effective amount of an additive composition for reducing the emissions of the distillate fuel comprising a homopolymer selected from the group consisting of polyisobutylene, polypropylene, hyperbranched polymers, and mixtures thereof, where the homopolymer has a molecular weight of from about 1600 to about 275,000.
 13. The distillate of claim 12 where the additive composition further comprises a component selected from the group consisting of an alkyl nitrate, a peroxide, and combinations thereof.
 14. The distillate fuel of claim 13 where the effective amount of the component ranges from about 100 to about 3000 ppm and the effective amount of the homopolymer ranges from about 20 to about 2500 ppm, both based on the total distillate fuel.
 15. The distillate fuel of claim 12 where the fuel has reduced NOx and/or particulate matter emissions as compared to an otherwise identical fuel absent the additive composition.
 16. The distillate fuel of claim 12 where the fuel has reduced NOx and particulate matter emissions are substantially the same or lower as compared to an otherwise identical fuel absent the additive composition.
 17. The distillate fuel of claim 13 where the component is an alkyl nitrate selected from the group consisting of 2-ethylhexyl nitrate (2EHN), iso-propyl nitrate, iso-amylnitrate, iso-hexylnitrate, cyclohexyl nitrate, dodecyl nitrate, diglycol nitrate and tetraglycol nitrate.
 18. A distillate fuel comprising: a hydrocarbon selected from the group consisting of diesel fuel, gasoline, jet fuel and kerosene; and an effective amount of an additive composition for reducing the emissions of the distillate fuel comprising a homopolymer selected from the group consisting of polyisobutylene, polypropylene, hyperbranched polymers, and mixtures thereof, where the homopolymer has a molecular weight of from about 1600 to about 275,000; and a component selected from the group consisting of an alkyl nitrate, a peroxide, and combinations thereof; where the fuel has reduced NOx and/or particulate matter emissions as compared to an otherwise identical fuel absent the additive composition.
 19. The distillate fuel of claim 18 where the effective amount of the component ranges from about 100 to about 3000 ppm and the effective amount of the homopolymer ranges from about 20 to about 2500 ppm, both based on the total distillate fuel.
 20. The distillate fuel of claim 18 where the component is an alkyl nitrate selected from the group consisting of 2-ethylhexyl nitrate (2EHN), iso-propyl nitrate, iso-amylnitrate, iso-hexylnitrate, cyclohexyl nitrate, dodecyl nitrate, diglycol nitrate and tetraglycol nitrate. 